This invention relates to glass manufacture, and in particular, to preheating of cullet utilized as a feedstock in a glass furnace.
Glass manufacturing is an energy-intensive process which requires about five-million Btu/ton for the melting process alone. As of the early 1980's, 300.times.10.sup.6 million Btu were consumed annually by the glass industry in the United States, with natural gas providing 70% of this energy.
In a typical glass furnace, the heat supplied to the melt is provided predominantly by natural gas mixed with preheated combustion air. The resultant flame fires over the melt and heat transfer to the melt is by radiation from the flame and furnace enclosure.
Additionally, some furnaces augment the heat with an electric boost. The major attraction of an electric boost is that it allows an increase in production from an existing furnace by providing an additional heat source to the melt. Typically, for a container furnace (a glass furnace for manufacturing bottles), an equivalent of 1200 kW or more can be added directly into the glass furnace by boosting, which yields a production increase of 20%.
In addition to energy costs, another significant factor in glass manufacturing is the cost of the constituents which are melted to form the glass. In recent years, the glass industry has been able to reduce this part of the cost by supplementing the traditional glass batch constituents with recycled glass, commonly referred to as cullet. The cullet is obtained from sources such as recycled bottles, which are broken up into practical sizes for use as glass forming constituents.
A method of increasing the amount of heat that can be added to a furnace while avoiding the high operating cost of an electric boost is to preheat the feedstock - i.e. the glass batch constituents and cullet. Preheating of glass batch has been proven feasible experimentally. However, the benefit from only preheating batch components will proportionally diminish as cullet use increases. As a result, cullet preheating offers an important alternative to an electric boost. Furthermore, due to furnace economics and state legislation encouraging the recycling of scrap glass, cullet use will very likely increase in the near future to the point where it may be the predominant feedstock in container furnaces in many parts of the country.
At present, the maximum amount of glass that can be pulled from an existing furnace is limited by the amount of energy that can be put into the furnace to melt the feedstock. This limit is reached when the furnace is at high fire resulting in maximum flow of the products of combustion.
As indicated above, additional energy can be put into the furnace by preheating the cullet so as to provide a more productive and efficient glass manufacturing process. One suggested method of preheating cullet is to use packed bed heating. A substantial disadvantage of packed bed heating, however, is that the heating gas tends to carve out a path for itself through the bed of cullet which results in uneven temperature distribution. Also, high pressure drop across a packed cullet bed would necessitate use of a substantial amount of auxiliary power to direct gases through the cullet. In addition, a very high residence time is needed to heat the cullet to the desired temperature.
Fluidized beds have been proposed for preheating glass batch and cullet. However, while fluidized beds are effective in heating glass batch, they are not well suited for use with glass cullet. The main problem is that the cullet size and its size distribution can be very large and the high cost to grind the cullet down to a consistent, workable distribution for use in a fluidized bed makes the process unattractive.
The benefits to be accrued by preheating glass cullet prior to the melting process are clear. Using a cullet preheater, it is desirable to heat the cullet to a temperature just below that at which it begins to become sticky and agglomerates. Tests have shown that with preheater inlet gas temperature of 1650.degree..degree.F., the cullet can be heated to 1100.degree. F. which is ideal. For example, for a furnace running on feedstock which is 70% by weight cullet, preheating cullet to a temperature of about 1100.degree. F. (a temperature just below that at which it begins to become sticky and agglomerates) can provide a productivity increase of as much as 30%.
In short, the glass industry is in need of a simple, cost-effective system, by which it can increase the productivity of furnaces by preheating its cullet feedstock prior to the melting process.
Therefore, it is an object of the present invention to provide a means by which existing glass furnaces can be made more productive and energy efficient.
A further object of the present invention is to overcome known production limitations of glass furnaces by delivering more energy to the furnace through cullet preheating.
Another object of the invention is to provide an improved method and apparatus for preheating cullet for use in a glass furnace.
Yet another object of the present invention is to provide a more energy efficient method for the manufacturing of glass which employs cullet as feedstock.